1. Field of the Invention
The present invention relates to a multiband radio technique using a plurality of radio frequency (RF) bands in a radio communication system, such as a cellular phone, a wireless LAN (WLAN), and the like. More particularly, the present invention relates to a radio communication apparatus which can be adapted to a plurality of RF bands.
2. Description of the Related Art
In recent years, an increase in the number of users of cellular phones has made it difficult to accommodate new users only in the frequency bands allocated for the third-generation cellular phones at first.
In order to solve this problem, it is necessary to develop a mobile terminal corresponding to a plurality of frequency bands, which are produced, for example, by allocation of a new frequency band, replacement of frequency bands that have been allocated to the second-generation, and the like.
As shown in FIG. 1, 3GPP, which has produced the specifications of the third-generation cellular phones, defines ten frequency bands between the band I and the band X.
In general, in the receiving section of a mobile telephone terminal, a direct conversion method is used, because the receiver using that method can be implemented with a fewer number of parts than the receiver using a superheterodyne method (for example, refer to Japanese Unexamined Patent Application Publication No. 2006-246323).
FIG. 2 is a diagram illustrating an example of a configuration of a communication apparatus having a receiving system of a cellular phone using a direct conversion method.
As shown in FIG. 2, the communication apparatus 1 has an antenna (ANT) 2, a switch (SW) 3, a duplexer (DUP) 4, a transmission power amplifier (PA) 5, a low-noise amplifier (LNA) 6, a filter 7, a local oscillator (LO) 8, a divider (phase shifter) 9, mixers (MIX) 10I and 10Q, low-path filters (LPF) 11I and 11Q, and a baseband circuit 12.
In the communication apparatus 1, an RF signal received by the antenna 2 is demodulated through the switch 3, the duplexer 4, the LNA 6, the filter 7, the mixers 10I and 10Q, the LPFs 11I and 11Q, and the baseband circuit 12.
Here, in FIG. 2, the LNA 6, the filter 7, and the mixers 10I and 10Q, which are surrounded by a broken line, are requested to have a different frequency characteristic depending on each receiving frequency.